This invention relates to printing blankets and especially to blankets that are continuous belts adapted for use on screen print machines.
Printing blankets used on screen print machinery are very large, and are enormously heavy. Frequently they are some 100 inches wide, and 100 yards long. Such blankets are formed in a closed loop. The material which is to be screened printed is glued to the blanket, and as each color step is imprinted on the goods, the blanket is advanced through one "repeat pattern". As a consequence, the entire blanket must start, move, and stop with an extraordinary degree of exactitude. If the variation in an entire traverse of the loop at any point exceeds 4 thousandths (0.004) of an inch, inferior printing will occur. If the amount of displacement as the blanket advances through one repeat pattern is greater than or less than in the preceding step, the pattern will not "fit".
This is serious, for "fit" in the textile printing sense means that each color, as it is successively applied, has been placed in the exact position required by the design. When fit is poor, some color margins overlap, the printed design on the finished goods appears "mushy", and sometimes unpleasant color mixing results. Poor fit results in severe economic loss, for the printed goods can then only be sold at sacrifice prices.
The blanket is generally engaged over and extends between two opposed rolls for the screen printing operation. One of the rolls is driven and drives the blanket through frictional engagement. The other roll is an idler roll. There are other commonly used methods of driving the blanket such as through the use of side clamps. The blanket is frequently guided by being engaged on its sides by opposing collar guides mounted on idler rolls. Even more abusive guides are sometimes used which do not rotate with the passing of the blanket but scrape against the blanket edges. It is necessary for the blanket to have a high degree of cross machine direction stiffness for these guides to work, a belt that is limp transversely will tend to buckle or bend during use when in contact with these guides and consequently will not align or track properly. Another reason requiring a blanket to have high transverse stiffness is the provision of sufficient stiffness to prevent the blanket's bending in the cross machine direction due to the contracting forces caused by the shrinkage of the fabric being printed when the fabric goes from dry to wet to dry during various printing stages.
So far as I am aware, transverse stiffness against transverse collapse, folding or distortion of printing blankets has in the past been achieved by providing thickness in the blanket. This thickness has many obvious disadvantages such as increasing the blanket's weight, depending on materials chosen increasing the blanket's cost and causing creping (wrinkling) in the material glued on the blanket if the blanket goes around a roll without first having the material removed.
By the present invention transverse stiffness against transverse collapse, folding or distortion has been provided in an expeditious way providing many improvements and also providing an enhanced ability to have good transverse stiffness in thinner blankets. Even in thicker blankets the blanket performance and life would in many instances be significantly improved by use of the present invention.
An excellent screen printing blanket is taught in U.S. Pat. No. 3,418,864, the contents of which are incorporated herein by reference. The manufacturing process taught therein is suitable for manufacturing the present preferred blanket with the exception that two spaced apart woven plys are those of the present invention containing the transverse rods. In addition in its preferred form the number of plys will also be reduced.